This invention relates to a process for removing dissolved silica from Bayer process solutions and agglomerated sodalite containing material for use therein.
In the Bayer process for producing alumina from bauxite, the bauxite containing aluminum trihydroxides or aluminum oxide-hydroxides is contacted with solutions containing caustic soda to dissolve the aluminum hydroxides as sodium aluminate while leaving most of the remaining constituents of the bauxite essentially unattached in solid form. A part or all of the silica content of the bauxite may also dissolve in the caustic soda solution to form a soluble sodium silicate. This reacts relatively slowly with the sodium aluminate in solution to form complex hydrated sodium aluminum silicates, known collectively as "desilication product", and are referred to hereinafter as either "desilication product" or "Bayer process desilication product". Desilication product from the Bayer process comprises one or more of the following compounds: Bayer Sodalite, 3(Na.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2.2H.sub.2 O)Na.sub.2 X, where X can be one of: CO.sub.3.sup.=, 2Cl.sup.-, SO.sub.4.sup.=, or 2AlO.sub.2.sup.- ; Cancrinite, Na.sub.6 Ca.sub.1.5 1.5Al.sub.6 Si.sub.6 O.sub.24 (CO.sub.3).sub.1.6 ; Noselite, 3(Na.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2.2H.sub.2 O)Na.sub.2 SO.sub.4 ; and Natrodavyne, 3NaAlSiO.sub.4.Na.sub.2 CO.sub.3. These desilication products are of low solubility in the resulting sodium aluminatecaustic soda solutions and largely precipitate out of solution thereby removing much of the undesirable silica from the solution phase.
After the digestion step for dissolving the aluminum hydroxide from the bauxite, the undissolved part of the bauxite, together with any desilication product that has precipitated at this point, which are known as "red mud", are separated from the solution, usually by filtration or sedimentation or both. The red mud is then disposed of, usually after being washed to recover the soluble valuables from the entrained caustic-aluminate solution. The clear caustic-aluminate solution after separation of the red mud is characterized by an alumina/caustic ratio grater than 0.55, where caustic is expressed in terms of equivalent Na.sub.2 CO.sub.3, and is commonly known as "pregnant liquor". It is subsequently cooled, diluted, seeded with aluminum trihydroxide crystals (gibbsite) and agitated for a period of time to precipitate a significant fraction of the dissolved alumina as gibbsite. This precipitate is then separated from the resulting spent liquor which is characterized by an alumina/caustic ratio between 0.3 and 0.4, and typically still contains in the order of half of the original dissolved alumina. A part of the separated gibbsite may be recirculated as seed material to the aluminum precipitation operation, while the remainder is washed to recover the soluble valuables from the entrained liquor, and is then suitably calcined to form alumina product of the Bayer process. The spent liquor may be reconcentrated, impurities removed and new caustic soda added as caustic feed to the digestion step.
Key parts of the Bayer process consist of the digestion step and the mud separation step in which the aluminum hydroxide materials of the bauxite are brought into solution in caustic-aluminate solution as soluble sodium aluminate and the remaining insoluble residue (red mud) is separated from the resulting pregnant solution, leaving a clear caustic sodasodium aluminate solution from which purified gibbsite can subsequently be crystallized. The nature of the solubility of the aluminum hydroxide minerals in caustic soda solutions usually requires that the digestion step be carried out at an elevated temperature in order to achieve higher solubilities of the alumina and hence reasonable liquor productivity (weight of alumina produced per volume of liquor circulated), while the precipitation step needs to be carried out at much lower temperatures to minimize the alumina solubility at this point in the process.
Most current Bayer plants make use of a digestion and mud separation module consisting basically of the equipment required to carry out the following sequence of operations:
(1) Preheating the incoming spent caustic aluminate liquor and bauxite passing to the digesters, using as much as possible recuperated heat followed by high-temperature heat from an external source;
(2) Carrying out the digestion while usually providing a residence time sufficient to permit removal of most of the silica dissolved from clay or quartz minerals in the bauxite by precipitation of a complex sodium aluminosilicate desilication product;
(3) Cooling the digested slurry by flashing the slurry at one or more decreasing pressures down to about atmospheric boiling temperature and using the flashed steam recovered for preheating purposes;
(4) At or below atmospheric boiling temperature, separating the red mud residue from the pregnant aluminate liquor, typically by filtration; or by flocculation, sedimentation and polish-filtration of the clear solution.
In most Bayer alumina plants, the major operating costs today in producing alumina are the costs related to bauxite and energy. Both of these costs are influenced by silica content of the bauxite, because silica reacts in the process. The reactive silica consumes caustic soda, so that high reactive silica bauxites are uneconomical to process. Bauxites with too low reactive silica contents may lead to high silica contents in the product alumina as a result of the limited desilication product formed as seed and the consequent high residual silica concentration in the pregnant liquor after digestion. Thus bauxites of a very limited range of reactive silica contents can be treated by the Bayer process, which tends to increase the price of suitable bauxites. A part of the silica in the Bayer liquors that does not precipitate during digestion, precipitates as scale on the heat transfer surfaces decreasing the efficiency of heat recuperation within the Bayer process, and hence increasing energy costs. An addition-al cost is the capital cost of spare equipment (heat exchangers, etc.) needed to allow for the frequent descaling of the heaters. Another important fraction of the dissolved silica is deposited on the alumina product during the precipitation, decreasing the purity of the alumina and its value.
In the past, control of dissolved silica has been effected mainly by means of controlling the temperature and time of the desilication step. In the North American variant of the Bayer process, desilication is commonly carried out concurrently with the digestion step. The temperature is set by alumina dissolution constraints and the only variable left for silica control is time by increasing the time under digestion conditions. This is costly in terms of capital equipment (pressure vessels) required, and in low temperature digests, additional holding time at 130.degree. to 150.degree. C. may lead to significant alumina losses due to reversion of dissolved alumina into solid form as the less soluble boehmite which is lost with the red mud solids. If adequate desilication cannot be achieved in this way, attempts are made to increase the amount of desilication product seed surface area to digestion by predesilicating or, in extreme cases, by adding clay minerals. These operations increase the costs by increasing both alumina and caustic soda losses.
When preliminary dilution of the digested slurry is used after a high caustic digest (European variant), it has been customary to hold the slurry for desilication at about the boiling temperature before separating the red mud. Again, holding time is the main control parameter and holding time again acts adversely as regards alumina losses. Marginal desilication product seed area control is possible by recirculating part of the red mud but at a further increased cost in alumina losses and equipment size.
It is possible to desilicate outside the digesters, in vessels open to the atmosphere. When the desilication takes place after the digestion step, it is known as post desilication. When desilication is carried out before the digestion step, on a slurry of bauxite and spent liquor, it is called predesilication. It is also possible to carry out the desilication of the spent liquor resulting after separation of the alumina product obtained during the precipitation step. The spent liquor is characterized by an alumina to caustic ratio of between 0.3 and 0.4, where the caustic concentration is expressed as equivalent Na.sub.2 CO.sub.3, and the total caustic concentration is between 200 and 350 g/L. The spent liquor is returned to the digestion step. All three types of desilication can be carried out in vessels open to the atmosphere. Such vessels are much less expensive to construct and to operate than the pressure vessels required for the digestion of bauxite. However, in the known process for pre and post-desilication at atmospheric pressure, the rate of desilication is very slow, because the desilication reaction rate is dependent on the surface area of the desilication product, and the surface area available is very low. In the known variants to the predesilication and post desilication processes, which are carried out in the presence of bauxite or red mud, attempts are made to increase the rate by adding a desilication product to the desilication vessels to increase the surface area. But added material because of its very fine particle size, usually much smaller than 100 microns, cannot be reused or recycled, because it is removed during the subsequent steps of separating red mud from the liquor. It is discarded after one use along with the red mud, making this route for improving desilication impractical.
Bayer sodalite, as described above, is an effective surface upon which to precipitate the desilication product. The problem has been to find a method whereby a large surface area of seed material, such as sodalite, can be introduced and maintained in the desilication vessels and recovered after use.
Domine et al, in U.S. Pat. No. 3,574,539 disclose that mordenite, an aluminosilicate of formula (Na.sub.2, Ca, K.sub.2) O.Al.sub.2 O.sub.3.10SiO.sub.2.6--7H.sub.2 O, can be made in an agglomerated form by adding a plasticizer chosen from a number of compounds including polyvinyl alcohol at a concentration between 1 and 10%, and then forming particulates by extrusion or pressing and drying. Upon stoving between 550.degree. and 800.degree. C., very hard particles are obtained. These particles are useful for the separation and desiccation of gases by preferential absorption. However, the polyvinyl alcohol binder is attacked by the strongly caustic solutions present in a Bayer process, making such material unsatisfactory for use in a Bayer process.
Various other desilication procedures are described in unsatisfactory in commercial operations. For instance, Raizman et al disclose in Komoleksn. Isool'z Miner Syr'ya 1986, (2) 52-5, that the addition of red mud and nepheline to an aluminate solution containing 400 g/L of SiO.sub.2 and autoclaving at 200.degree. C. resulted in the desilication of the solution. Another procedure is that of Khanamirova et al reported in Kompleksn. Ispol'z Miner Syr'ya 1980, (2) 52-6, and 1980 (3) 67-71 in which sodium aluminate solutions are desilicated by autoclaving in the presence of MgSiO.sub.3. In a method described by Turinskii et al in SU 1034995, dissolved silica is removed through the formation of insoluble hydrogarnet (calcium aluminate silicate hydrate), by addition of lime; since only a small amount of silica is incorporated into the hydrogarnet, the method is expensive in terms of the ratio of lime used and alumina lost to the amount of silica removed. SU 1034995, by Turinskii et al, discloses the use of an active calcium hydroxide in lime milk to achieve the low temperature desilication of Al solutions by the hydrogarnet route. Yet another technique is described by Derdacka-Grzymek in PL 131,922 in which sodium aluminate solutions are desilicated by the addition of lime at 98.degree. C., followed by filtration to remove the precipitated silica impurity.
There still remains a need for an efficient and inexpensive procedure for removing dissolved silica from Bayer process solutions.